Photoreceptors based on hydrogenated amorphous silicon (a-Si:H) are currently used in commercial xerographic photocopy machines and laser printers. A photoreceptor of this type has a surface that is charged by a corona discharge. Charge acceptance refers to the maximum charge that may be induced on the surface by corona discharge. An image of an item to be copied is projected onto the surface. Exposure of the photoreceptor to the image causes selective discharge of the surface caused by photoconductivity in the photoreceptor. An electrostatic image is thereby formed. The image is developed by attaching toner particles to the charged surface regions of the photoreceptor and then transferring the toner to paper or other suitable material where it is fused by heat or pressure to produce a permanent image.
The surface voltage of a photoreceptor is subject to discharge over time even in the absence of light. This is known as dark discharge. Thermal generation of bulk carriers, injection from the back contact and dielectric breakdown contribute to dark discharge. A low dark current facilitates the holding of the surface charge. High photoconductivity is desirable for allowing rapid discharge in the presence of light.
Referring to FIG. 1, a xerographic photoreceptor 10 includes a hydrogenated amorphous silicon photoconductor layer 12, a hydrogenated amorphous silicon nitride or silicon carbide surface/passivation layer 14, a dielectric blocking layer 16 and a metal (e.g. aluminum) substrate 18. The dielectric blocking layer 16 (e.g. p+a-Si:H) helps suppress injection from the back contact (i.e. the substrate 18). Light boron doping of the photoconductor layer 12 (p− a-SiH) reduces bulk thermal generation of carriers. The surface layer 14 helps reduce surface conduction and thereby prevents the blurring of the latent image. It also serves to protect the a-Si:H surface from scratching.